X-ray Variability and Period Determinations in the Eclipsing Polar DP Leo

An analysis of ROSAT observations for the eclipsing magnetic cataclysmic binary DP Leo provides constraints on the origin, size, temperature, variability and structure of the soft X-ray emission region on the surface of the white dwarf. These data, when combined with prior observations, show a progression of approximately 2 degrees per year in the impact position of the accretion stream onto the white dwarf. One explanation for the observed drift in stream position is that a magnetic activity cycle on the secondary produces orbital period oscillations. These oscillations result in an orbital period which cycles above and below the rotational period of the nearly synchronous white dwarf. The accretion stream and X-ray emission regions are modeled to fit the observational data. A distance to the system is also calculated. [An erroneous value for the cyclotron luminosity, included in an earlier paper version of the preprint, is corrected here.]Comment: uuencoded PostScript file (25 pages) + 8 figures available by anonymous ftp to ftp.astro.psu.edu (in the directory /pub/robinson), to appear in ApJ, PSU preprint 1994-1

Math modeling for helicopter simulation of low speed, low altitude and steeply descending flight

A math model was formulated to represent some of the aerodynamic effects of low speed, low altitude, and steeply descending flight. The formulation is intended to be consistent with the single rotor real time simulation model at NASA Ames Research Center. The effect of low speed, low altitude flight on main rotor downwash was obtained by assuming a uniform plus first harmonic inflow model and then by using wind tunnel data in the form of hub loads to solve for the inflow coefficients. The result was a set of tables for steady and first harmonic inflow coefficients as functions of ground proximity, angle of attack, and airspeed. The aerodynamics associated with steep descending flight in the vortex ring state were modeled by replacing the steady induced downwash derived from momentum theory with an experimentally derived value and by including a thrust fluctuations effect due to vortex shedding. Tables of the induced downwash and the magnitude of the thrust fluctuations were created as functions of angle of attack and airspeed

Morphometric approach to many-body correlations in hard spheres

We model the thermodynamics of local structures within the hard sphere liquid at arbitrary volume fractions through the \textit{morphometric} calculation of $n$-body correlations. We calculate absolute free energies of local geometric motifs in excellent quantitative agreement with molecular dynamics simulations across the liquid and supercooled liquid regimes. We find a bimodality in the density library of states where five-fold symmetric structures appear lower in free energy than four-fold symmetric structures, and from a single reaction path predict a relaxation barrier which scales linearly in the compressibility factor. The method provides a new route to assess changes in the free energy landscape at volume fractions dynamically inaccessible to conventional techniques.Comment: 6+17 pages, 3 figure

A procedure for assessing aircraft turbulence- penetration performance

Subsonic transport aircraft performance assessment during atmospheric turbulenc

Comparative analysis of techniques for evaluating the effectiveness of aircraft computing systems

Performability analysis is a technique developed for evaluating the effectiveness of fault-tolerant computing systems in multiphase missions. Performability was evaluated for its accuracy, practical usefulness, and relative cost. The evaluation was performed by applying performability and the fault tree method to a set of sample problems ranging from simple to moderately complex. The problems involved as many as five outcomes, two to five mission phases, permanent faults, and some functional dependencies. Transient faults and software errors were not considered. A different analyst was responsible for each technique. Significantly more time and effort were required to learn performability analysis than the fault tree method. Performability is inherently as accurate as fault tree analysis. For the sample problems, fault trees were more practical and less time consuming to apply, while performability required less ingenuity and was more checkable. Performability offers some advantages for evaluating very complex problems

Public Perceptions of Wisconsin’s Pavements and Tradeoffs in Pavement Improvement

Findings are reported from Phase II of a three-phase pooled-fund project in Wisconsin, Iowa, and Minnesota to determine perceptions of drivers regarding pavement of rural two-lane highways. Among the survey topics were drivers\u27 trust in the state department of transportation (DOT), pavement improvement trade-offs, and pavement evaluation. Results of the Wisconsin portion of the survey data are the focus of this study. The survey questionnaire was based in part on Phase I focus groups conducted to gauge beliefs about pavements as well as the language describing ruts, tining, and other pavement characteristics. Phase II entailed a statewide telephone survey of at least 400 randomly selected drivers in each of the three states. Although the focus here is on Wisconsin results, survey responses across the three states were very consistent. Included in the findings discussed are perceptions of pavement and the state DOT and pavement improvement options relating to construction, travel time, and delays. Results disclose key public perceptions of priorities with regard to spending limited funds. Also discussed are statistically significant relationships providing additional insights into public perceptions and pavement improvement on rural two-lane highways

Some Physical Consequences of Abrupt Changes in the Multipole Moments of a Gravitating Body

The Barrab\`es-Israel theory of light-like shells in General Relativity is used to show explicitly that in general a light-like shell is accompanied by an impulsive gravitational wave. The gravitational wave is identified by its Petrov Type N contribution to a Dirac delta-function term in the Weyl conformal curvature tensor (with the delta-function singular on the null hypersurface history of the wave and shell). An example is described in which an asymptotically flat static vacuum Weyl space-time experiences a sudden change across a null hypersurface in the multipole moments of its isolated axially symmetric source. A light-like shell and an impulsive gravitational wave are identified, both having the null hypersurface as history. The stress-energy in the shell is dominated (at large distance from the source) by the jump in the monopole moment (the mass) of the source with the jump in the quadrupole moment mainly responsible for the stress being anisotropic. The gravitational wave owes its existence principally to the jump in the quadrupole moment of the source confirming what would be expected.Comment: 26 pages, tex, no figures, to appear in Phys.Rev.